In exploration for oil columns, for example offshore (subsea) oil columns, when a potential column has been identified, (ie. using standard geological exploration techniques), it is usual practice to test-drill the site to ascertain whether the site contains oil (or could contain oil other than at the point of drilling).
Test drilling is typically expensive and is performed by stringing together pieces of pipe and drilling through, usually, a shale crust until a reservoir (eg. of sandstone or limestone) suspected of containing oil is reached. The rate of penetration of drilling increases when such a reservoir is reached, and this signifies that testing for oil in the reservoir can commence. Mud and cuttings from the drilling operation eventually reach the drill rig at the ocean surface (eg. in offshore exploration) and these are analysed in order to determine the presence or absence of oil. The techniques of cuttings analyses are collectively referred to as "shows". Known show techniques include detecting hydrocarbon gases in the mud by flame ionisation or thermal conductivity, detecting oil fluorescence by ultraviolet illumination of the mud, and oil stain, oil odour and oil cutting (adding of solvent) techniques etc. However, the existing techniques can be subject to false and/or ambiguous results. For example, heavy mud used to seal around the drill string during drilling may include hydrocarbon and this can also reach the surface during testing giving false results.
It would be advantageous if a show technique could be provided that reliably indicates oil presence (or absence) in a reservoir, either now or in the past, for example, during test-drilling and/or drill site re-exploration to assist in the location of an oil column.